It is known that magnetic field sensors can be made based on anisotropic magnetoresistance (AMR) or giant magnetoresistance (GMR) elements. However, several properties of AMR and the more sensitive GMR sensor structures may still be a limiting factor for some types of applications. Firstly, the AMR effect and the GMR effect both occur in metallic thin films, resulting in a relatively low sheet resistance. As in the device geometry used the current is directed parallel to the plane of the film it is advantageous from the point of view of a large output voltage to fabricate the element(s) in the form of a long meandering stripe pattern. However, this then results in a rather large sensor area (up to a few mm2 typically). Another negative consequence of the use of a meandering pattern is that this reduces the efficiency of the possible use of a flux concentrator. Secondly, proper functioning of AMR and GMR sensors requires the deposition of additional patterned structures, thereby complicating the manufacturing process. AMR sensors require the deposition of a so-called barber pole overlay to linearize their resistance versus field curve. A factor that complicates the fabrication of both AMR and GMR sensors is that their proper functioning (no zero-field offset) requires the use of an external coil or of integrated conductors (insulated from the sensitive part of the structure) for flipping the magnetization direction of the magnetically sensitive layers.
The present invention provides a novel class of magnetic field sensors, advantageously making use of magnetoresistive spin tunneling junctions in order to realize sensors with improved sensitivity, miniaturizability and simplicity of design. According to the invention, a magnetic field sensor is characterized in that the sensor comprises a first common, elongated ferromagnetic layer F1, with an in plane easy magnetization axis normal to the elongation direction of said layer F1, and two further ferromagnetic layers F2 with opposite magnetization directions, situated at some distance from each other in the longitudinal direction of layer F1, forming a pair of magnetoresistance elements of the spin tunnel junction type having an electrical barrier layer sandwiched between the ferromagnetic layers F1, F2, and means to measure resistance across the spin tunnel junctions. Such a sensor enables magnetic fields to be measured, in operation, which are applied in the plane of the layer F1 and normal to the easy axis direction of the layer F1 (i.e. in the direction of elongation of layer F1). In preferred embodiments two crossing layers F1 and F2 are used, forming two pairs of tunnel junctions, thus forming a simple, yet very sensitive Wheatstone bridge arrangement.
The elongated form of the layer F1 also offers the advantage that the flux of the field to be measured is concentrated at the magnetoresistance elements of the spin tunnel junction type, thus increasing the sensitivity of the design. The sensor area can be reduced. The design of the F1 layers can be further optimized to increase flux concentration, for instance by choosing the width of the F1 layer so as to be smallest close to the magnetoresistance element(s) and larger at a greater distance from said magneto-resistance element(s).